Many semiconductor devices, including but not limited to optoelectronic devices such as photovoltaic cells, feature one or more layers of crystalline semiconductor materials epitaxially grown or deposited on a substrate. It may be desirable to facilitate subsequent processing steps or reduce overall substrate costs to epitaxially grow a crystalline layer or layers on a parent substrate and then remove the layers from the parent substrate for processing or association with a secondary substrate or superstrate.
Epitaxial growth of various types of semiconductor materials having desired characteristics may be readily accomplished when the epitaxial layers are lattice matched to the parent substrate or when subsequent layers are substantially lattice matched to underlying layers. Many semiconductor materials however, which have desired physical, optical or electrical characteristics are not inherently lattice matched with the substrates or underlying layers having attributes necessary for a device to function. Accordingly, a much broader range of semiconductor alloys is available for device design and fabrication if useful layers of lattice-mismatched semiconductor materials can be epitaxially grown and then processed into useful devices.
Excessive lattice mismatching in adjacent crystalline materials causes excessive lattice strain, which, when present to a sufficiently high degree, is usually manifested in the formation of dislocations, fractures, and other problems that degrade or destroy the electrical characteristics and capabilities of a device. Lattice mismatching may occur between an epitaxial layer and a parent substrate or between adjacent epitaxial layers. If the lattice-mismatched layers are carefully prepared from selected materials however, electrically useful layers can be fabricated.
Typically, the problems associated with extensive lattice mismatching can be controlled by growing compositionally graded layers, where the lattice-mismatch gradient is maintained below a critical value, for example less than 1% lattice mismatch per micron. Proper graded layer growth is sufficient to prevent excessive dislocations or fractures leading to a rough and unusable layer. However, graded lattice-mismatched layers will still have some degree of dislocation development. It is important to note that graded layers will not be fully relaxed by dislocation development. On the contrary, internal stress will still be present which will result in some degree of residual strain within the layers.
Alternatively, the problems associated with excessively lattice-mismatched layers, such as fractures and excess dislocations, can be mitigated by pseudomorphically growing one or more epitaxial layers to a thickness that is less than a critical thickness of the selected semiconductor alloy. In particular, a relatively thin mismatched epitaxial layer can be grown without excessive dislocation formation on an underlying layer provided the material parameters (e.g. layer thickness, elastic coefficients, etc.) are properly selected to maintain a coherent interface between the two layers under specific growth conditions. The term “coherent interface” is defined herein as an interface where an overlying epilayer takes on the same lattice constant as the underlying layer by elastic deformation, thus providing an interface which is functionally lattice matched, even though the respective lattice constants for each of the materials in bulk form may be substantially different.
Although the maintenance of a coherent interface may reduce or eliminate large scale problems such as fractures or dislocations, the elastic deformation for maintaining a coherent interface between lattice-mismatched epilayers may result in stress and compressive or tensile strain within the epilayers. Similarly, thicker lattice-mismatched layers prepared using a graded layer approach may have residual strain within the layers as discussed above.
This strain may cause bowing or curling of an epiwafer, which includes both the parent substrate and grown epilayers. In addition, the strain within lattice-mismatched epilayers may cause the epilayers to bow or curl if the parent growth substrate is selectively removed. Accordingly, flexible lattice-mismatched epistructures inherently settle into a more or less complex bowed state. Bowing or curling as used herein is defined broadly to encompass varying degrees of offset between the edges and center of a flexible epistructure. The degree of bowing is a function of epistructure area, the level of strain and other effects such as non-isotropic relaxation or thermal expansion.
Bowing may also occur in fully lattice-matched epistructures, particularly if the lattice-matched epistructures are removed from the parent substrate. Bowing in lattice-matched epistructures may be the result of thermal effects (e.g. differing coefficients of thermal expansion between layers) or stress induced by post-epitaxy processing steps such as metallization.
Epiwafer or epistructure bowing creates significant problems for device fabrication and device implementation. For example, it may be difficult to perform accurate lithography or other processing steps on a substantially non-flat, bowed, epiwafer or epistructure.
A typical epitaxially grown epistructure is deposited on a flat parent substrate. Generally, the parent substrate is substantially rigid, although it may be subject to bowing as described above. Certain devices may benefit from the implementation of an epistructure in a non-flat, for example cylindrical device. Highly curved epistructures are difficult to prepare directly.
The embodiments disclosed herein are intended to overcome one or more of the limitations described above. The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.